Direction of expression differences. The mean (± 2 s.e.) of log2 transformed fold-change in expression level in response to the absence of the F-plasmid are plotted for two different E. coli strains, DH5α and MG1655. While "plasmid effect only" genes showed the expected change in one direction only (expression in the presence of the F-plasmid), the "bacterial and plasmid additive effects only" genes and "bacterial/plasmid interaction effect" genes were unbiased (i.e.: the mean change across genes was close to zero).